Ball bearings used in cryogenic applications have to function at temperatures of about −130° C. to about −270° C. (about −200° F. to about −450° F.). For example, such ball bearings are used in pumps that are submerged in liquefied natural gas (LNG), or in turbo-pumps of rocket engines burning liquid hydrogen (LH2) with liquid oxygen (LOX). The bearings comprise balls held in place by cages and normally run at high speeds and carry significant loads.
The robustness of a ball bearing is significantly reduced in cryogenic applications relative to the performance of the ball bearing at room temperature. For example, materials can become quite brittle in cryogenic applications, necessitating a variety of design reinforcement measures such as the addition of metallic shrouds, side plates, riveting, etc, when one-piece cage designs are used. A common failure mode of ball bearings in cryogenic applications is failure of the bearing cage. The significant differences between the coefficient of thermal contraction (CTC) of metallic bearing rings, metallic cage reinforcement components, and non-metallic cage materials further add to the complexity of designing a bearing for cryogenic use.
Pumps used for cryogenic aerospace applications, and the bearings therein, are not only subject to the severity of cryogenic temperatures, but also to limitations on pump weight. In addition, no active lubrication of the bearings in the usual sense is available or feasible. One lubrication effect provided to a bearing in such a pump, if any, is often limited to the result of a bypass flow of the cryogenic fluid (for example, LNG, LH2 or LOX) through the bearing. However, such bypass flow is restricted due to limited clearances within the bearing.
A bearing cage material can serve as a source of limited “transfer lubrication” in cryogenic bearings. Cage materials made from Teflon®-based composite materials such as Armalon™ and Rulon™ have been used in severe cryogenic applications. In relatively less severe applications, such as pumping LNG, phenolic composites have been employed as materials for cages, with mixed results.
It is an object of this invention to provide a cryogenic bearing (that is, a bearing for cryogenic applications) that improves upon prior art cryogenic bearings.